1. Field of the Invention
The present invention relates to novel viscoelastic viscosurgical materials and ophthalmic surgical techniques utilizing the materials.
2. Prior Art
In ophthalmic surgical procedures such as intraocular lens implantation, cataract surgery, retinal detachment repair, etc., there exists a need for viscous, gel-like compositions to fill the chambers of the eye to protect sensitive tissue such as the corneal endothelium from trauma, see U.S. Pat. No. 4,141,975; Pruett et al, Arch. Ophthalmol., 97:2325 (1979); Holmberg et al, Ophthalmology, 91:45 and 53 (1984); Pope et al, ophthalmology, 87:669 (1980): Mac Rae et al, Am. J. Ophthalmol, 13:811 (1981).
The most commonly employed materials are solutions of hyaluronic acid (HA), chondroitin sulfate (CS) and methylcellulose (MS). HA has been the most widely used and appears to owe its unusual rheological and viscoelastic solution properties to its polyanion polyelectrolyte molecular structure.
However, HA is extremely expensive. Furthermore, it requires extraordinary purification to remove as much proteinaceous immunogenic material as possible but still may provoke immune reactions in some patients. Its use is also often accompanied by significant undesireable intraocular pressure rise which necessitates washing HA for the eye at the end of surgery and may also require antiglaucoma therapy.
Chondroitin sulfate solutions do not inhibit pseudoplastic behavior, i.e., the viscosity is relatively constant at all shear rates. Accordingly chondroitin sulfate solutions do not exhibit the same degree of anterior chamber support as pseudoplastic fluids such as those prepared using sodium hyaluronate. Furthermore, since the viscosity of the chondroitin sulfate solutions does not decrease at increasing shear rates (as do pseudoplastic materials) extremely high pressures are needed to apply or irrigate chondroitin sulfate solutions through a syringe (Mac Rae et al; "The Effects Sodium Hyaluronate, Chondroitin Sulfate, and Methyl Cellulose on the Corneal Endothelium and Intraocular Pressure, Am. J. of Ophthalmology, 95:332-341 (1983). Additionally, Commercially available chondroitin sulfate solutions (20 to 50 percent solutions) have osmolarities in excess of 500 mOsm. Such high osmolarities are detrimental to the corneal endothelium. Lastly, as reported by Mac Rae et al, in the American Journal of Ophthalmology, supra, 20 percent chondroitin sulfate may cause a sharp increase in intraocular pressure in the first one to four hours after intracameral injection and, thererfore, anterior chamber washout is indicated.
It is an object of the present invention to provide improved ophthalmic viscoelastic surgical materials and ophthalmic surgical thecniques embodying same which are not subject to the above-noted disadvantages.